Earth-boring tools are used to form boreholes (e.g., wellbores) in subterranean formations. Such earth-boring tools include, for example, drill bits, reamers, mills, etc. For example, a conventional fixed-cutter earth-boring rotary drill bit may include a bit body having generally radially projecting and longitudinally extending blades. A plurality of cutting elements may be positioned on each of the blades. Generally, the cutting elements have either a disk shape or a cylindrical shape. The cutting elements may include a layer or “table” of superabrasive material, such as polycrystalline diamond, formed on or attached to a supporting substrate made from a hard material, such as cemented tungsten carbide. Such cutting elements may be referred to as “polycrystalline diamond compact” (PDC) cutting elements. The PDC cutting elements may be secured to the body of an earth-boring tool, such as a drill bit, within cutting element pockets, or recesses, formed in rotationally leading surfaces of each of the blades. Conventionally, the cutting elements are secured in the recesses of the bit body by a braze alloy.
The bit body of a fixed-cutter earth-boring rotary drill bit may be made from, for example, a metal alloy (e.g., steel) or a particle-matrix composite material such as cemented tungsten carbide. Steel bit bodies are generally fabricated using standard machining processes. Particle-matrix composite bit bodies are typically manufactured by forming a graphite mold having a mold cavity therein, filling the cavity of the graphite mold with tungsten carbide particles, and infiltrating the tungsten carbide particles with a molten metallic alloy such as bronze. The molten alloy infiltrates through the tungsten carbide particles, after which the molten alloy cools and solidifies to form a continuous matrix phase in which the tungsten carbide particles are cemented. The surfaces of the graphite mold within the cavity may include recesses or pockets formed by, e.g., milling and/or drilling, and displacements may be inserted therein. The displacements may include, for example, graphite, silica, or another material that will withstand the temperatures of the casting process, but that can also be subsequently removed from the bit body as discussed below. The displacements may extend into the interior of the mold cavity to create cutting element pockets in the bit body corresponding to the size and shape of the cutting elements to be installed in the bit body. The displacements may be sized to provide a small annular clearance between the cutting element and the pocket to facilitate fixing the cutting elements into the recesses of the bit body.
After the bit body is formed and the graphite mold is removed, the displacements are removed from the cutting element pockets of the bit body, and cutting elements are brazed into the cutting element pockets. In the brazing process, a cutting element is positioned within a cutting element pocket, and molten braze material is applied to the interface between the cutting element and the pocket in the bit body. The cutting element is rotated within the pocket to distribute the molten braze material around the circumference of the cutting element and over the entire interface between the cutting element and the surfaces of the bit body within the pocket. The braze material is then allowed to cool and solidify to secure the cutting element within the pocket. Worn or failed cutting elements may be replaced in bit bodies by heating and melting the braze material to release the cutting element from the pocket. The pocket then may be cleaned and repaired as needed, after which a new cutting element may be secured to the bit body within the pocket using the brazing method described above.
Fixed-cutter earth-boring rotary drill bits may be configured for use in different formation materials by altering various parameters, such as the size and shape of the cutting elements, the number of cutting elements, and the orientation at which each cutting element is carried by the bit body with respect to the formation. For example, each cutting element may be oriented such that a cutting face of the cutting element forms a particular angle with a plane tangent to the formation at the location of contact with the cutting element. This angle is commonly referred to as a “back rake angle.” Cutting elements with different back rake angles may be suited for engaging different formation materials. Generally, cutting elements oriented at low back rake angles are more aggressive and remove formation material at a relatively higher rate, while cutting elements oriented at high back rake angles are less aggressive, and remove formation material at a relatively lower rate. Cutting elements oriented at aggressive low back rake angles, however, may wear and suffer damage quickly when drilling through hard formations. Thus, cutting elements with less-aggressive high back rake angles may be well-suited for use in relatively hard formation materials such as limestone, dolomite, sandstone, etc., as they allow sufficient cutting action without causing substantial, rapid degradation to the cutting elements. Conversely, cutting elements with aggressive low back rake angles may be better suited for drilling through relatively softer formations, as they may be used to drill at a higher rate-of-penetration (ROP) without experiencing substantial wear and degradation.
The back rake angle of the cutting elements is conventionally established by adjusting the orientation of the cutting element pockets formed in the bit body into which the cutting elements are affixed. Thus, unique bit bodies must be designed and fabricated for efficient drilling into formations of differing hardness.